1. Field of the Invention
The present invention relates to an exhaust processing method, and a plasma processing method and apparatus, and more specifically, to an exhaust processing method, and a plasma processing method and apparatus which are used to execute plasma processing on substrates or films by a plasma CVD apparatus or a sputtering apparatus for forming films or by a dry etching apparatus for processing deposited films during a process of fabricating semiconductor devices.
2. Related Background Art
Plasma processing is generally and widely used as a method of using energy such as electromagnetic waves, heat, or light to excite a raw material gas to obtain plasma and exposing a predetermined substrate to the plasma to deposit a film thereon or execute doping, etching, or the like.
For example, the plasma CVD process comprises introducing a raw material gas into a plasma processing chamber, reducing the pressure in the plasma processing chamber by means of an exhaust pump, and applying a direct current, a high-frequency wave, or microwave power to the raw material gas to ionize, dessociate, or excite it into plasma to thereby form a deposited film on a substrate. The plasma CVD process conventionally uses parallel plate electrodes as well as glow discharge or RF discharge using high frequency.
In addition to the discharge process using parallel plate electrodes, a process of decomposing a compound gas and depositing a film by means of thermal energy has been used. The process using heat energy includes the Hot Wall process of using a gas with a relatively low decomposition temperature such as Si2H6 as a raw material and heating the plasma processing chamber itself to decompose the gas and the thermal CVD process of obtaining a similar effect to the above process by heating a substrate. Furthermore, there is the hot wire CVD process comprising depositing a thin film using a metallic filament such as a tungsten filament which is heated beyond the melting point of silicon crystals. Additionally, there is the optical CVD process comprising decomposing a raw material gas to form a deposited film by irradiating a substrate surface with light such as ultraviolet rays.
The dry etching process is general as a deposited-film processing method for, after forming a deposited film such as an amorphous semiconductor film, a microcrystal semiconductor film, or an insulating film, processing the film into a desired pattern or thickness.
To form a silicon-based amorphous or microcrystal semiconductor film, a raw material gas such as SiH4, Si2H6, SiF4, or Si2F6 is used. A doping gas such as BF3, B2H6, or PH3 is used. Further, to form a silicon germanium-based amorphous film or microcrystal film, a GeH4 gas, in addition to the above gases, is often used as the raw material gas.
The (plasma) pressure in the plasma processing chamber must be about 1.3×101 Pa to 1.3×103 Pa in order to supply power ranging from DC to high frequency. It must be 1.3×10−1 Pa to 1.3×102 Pa in order to supply microwave power. Additionally, the substrate is heated at 200 to 400° C.
FIG. 2 shows a schematic sectional view of a plasma CVD apparatus as one of representative prior art deposited-film forming apparatuses. With reference to FIG. 2, an example will be described in which an amorphous silicon film is produced by means of a general plasma CVD process using a high frequency. In FIG. 2, reference numeral 1 denotes a plasma processing chamber, reference numeral 2 denotes an exhaust means (a rotary pump and a mechanical booster pump), reference numeral 3 denotes an exhaust line, reference numeral 4 denotes a conductance adjusting valve, reference numeral 5 denotes a power applying electrode, reference numeral 6 denotes a high-frequency power source, reference numeral 7 denotes a high-frequency introducing section, reference numeral 8 denotes a substrate, reference numeral 9 denotes a substrate holder, reference numeral 10 denotes gas introducing section, reference numeral 11 denotes a pressure gauge, reference numeral 12 denotes a discharge region, and reference numeral 15 denotes an exhaust line heater.
The substrate 8 is fixed to the substrate holder 9, a substrate access port (not shown) of the plasma processing chamber 1 is closed, and the exhaust means 2 is used to exhaust the plasma processing chamber 1 to thereby reduce the pressure therein. The substrate 8 is heated to a temperature that meets plasma processing conditions, by means of a substrate heater (not shown) fixed to the substrate holder 9. A plurality of deposited-film-forming raw material gases (SiH4, Si2H6, H2, a doping gas) supplied from gas cylinders (not shown) at flow rates controlled via a gas flow controllers (not shown) are mixed together and supplied to the discharge region 12 in the plasma processing chamber 1 through the gas introducing section 10. A high frequency from the high-frequency power source 6 is applied to the power applying electrode 5 to induce discharging in the discharge region 12 between the power applying electrode 5 and the substrate 8 and substrate holder 9 which are located opposite to the power applying electrode 5 and acting as a substrate electrode.
The gas in the plasma processing chamber 1 is discharged by the exhaust means 2 through the exhaust line 3 so as to be constantly replaced with a newly supplied gas. The pressure in the discharge region 12 is monitored by the pressure gauge 11 so that, based on a resulting pressure signal, the opening degree of a conductance adjusting valve 4 provided in the path of the exhaust line 3 is adjusted to control the pressure in the discharge region 12 at a constant value. The deposited-film-forming raw material gas is ionized, dessociated, or excited in plasma induced in the discharge region 12, to form a deposited film on the substrate 8.
The conductance adjusting valve 4 is useful for adjusting the pressure to a desired value regardless of the flow rate of the raw material gas. The conductance adjusting valve 4 increases or decreases exhaust conductance by varying the cross section of the exhaust line 3.
After a deposited film has been formed, the supply of the raw material gas is stopped, and a purge gas (He, Ar, or the like) is newly introduced to sufficiently substitute for the raw material gas remaining in the plasma processing chamber 1 or the exhaust means 2. After the purging has been completed and the plasma processing chamber 1 has then cooled down, the pressure is returned to the atmospheric pressure and the substrate 8 is then removed.
The exhaust line heater 15 provided on the exhaust line 3 extending from the plasma processing chamber 1 to the exhaust means 2 increases the temperature of the exhaust line 3 to cause the decomposition and reaction of byproducts before they are removed.
The byproducts as used herein refer to powders which are generated in the plasma under discharge conditions (the pressure, gas flow rate, and power value) when a SiH4-based gas is used and which adhere to or deposit on the electrode, the substrate holder, a wall of the chamber or exhaust line, or the surface of the valve. Conventionally, the byproducts have been removed by a method in which a temperature is elevated using the exhaust line heater 15 to cause the decomposition and reaction of the byproducts.
Further, Japanese Patent Application Laid-Open No. 8-218174 discloses a method of providing a trap on the exhaust line, and precipitating and coagulating byproducts on the trap while heating the location between the plasma processing chamber and the trap to prevent the byproducts from adhering to the exhaust line wall. Japanese Patent Application Laid-Open No. 7-130674 discloses a process of providing opposite electrodes on the trap on the exhaust line and inducing discharging to deposit an unreacted gas and the byproducts on the surface of a wall of the trap as a hard film. Japanese Patent Application Laid-Open No. 4-136175 discloses a process of reducing the amount of unreacted gas by using a reaction chamber in which plasma is induced to cause the reaction of an unreacted gas to thereby form a film.
Problems with a plasma processing apparatus for forming (processing) a deposited film by means of plasma processing are the adverse effects on film quality due to the mixture of the byproducts into the deposited film, the byproducts being generated during plasma processing and adhering to or depositing on locations other than the substrate, as well as the necessity of maintenance for the apparatus for removing the byproducts adhering to or depositing on the exhaust line or the valve.
The byproducts attached to the inside of the plasma processing chamber sucks the gas or whirls inside it and may be caught in the deposited film on the substrate as dusts or contamination to adversely affect the characteristics of the deposited film.
Further, the byproducts attached to the exhaust means may significantly increase the viscosity of a pump oil when the exhaust means comprises a rotary pump or may adhere to rotors, which may contact with each other to cause malfunction, when the exhaust means comprises a mechanical booster pump or a dry pump. Additionally, as described previously, when the byproducts attached to the exhaust pipe wall or valve grow to reduce the effective cross section thereof, the exhaust conductance may decrease gradually, thereby making it impossible to obtain a desired discharge pressure in the plasma processing chamber. Furthermore, the conductance adjusting valve may malfunction.
In FIG. 2, as described previously, the exhaust line heater 15 is used to increase the temperature in the exhaust line 3 to remove the byproducts by causing decomposition or reaction thereof. It is difficult, however, for this process to sufficiently raise the temperature in the exhaust line, the pressure of which has been reduced, that is, this process is thus insufficient as a method for removing the byproducts.
The dry etching process is also known as the method for removing the byproducts. The dry etching process comprises inducing discharging in the plasma processing chamber to etch the byproducts in the exhaust line by means of radicals of an etching gas with a long lifetime or inducing discharging in the exhaust line for etching. The etching, however, requires considerations for the corrosiveness of the plasma processing chamber members, the exhaust line member, and the exhaust pump, and for the effects of etching residues or byproducts as contamination during the formation (processing) of the deposited film.
Another conventional process comprises providing a trap in the exhaust line, arranging parallel plate electrodes inside the trap, and decomposing and depositing an unreacted compound gas in the trap using glow discharge or high-frequency-based RF discharge. The unreacted compound gas, however, is decomposed and deposited on the surface of a wall of the trap at a low rate, so that the byproducts may disadvantageously be transported to the exhaust pump. Further, arrangement of the parallel plate electrode inside the trap requires a certain amount of space, resulting in no degree of freedom for the installation of the trap.
Another conventional process comprises installing a heating element inside the trap to directly heat the interior of the exhaust line. This process is effective on the removal of the byproducts, but plasma that has grown up to the interior of the trap may not only decompose the byproducts but also generate them, thereby dispersing the effects of the heating element.
Although the plasma CVD process is more often used for industrial purposes, that is, to produce semiconductor films, there is a demand for a further increase in the area of the film formed and in the amount of time spent in film formation and thus an associated increase in the amount of byproducts deposited in the exhaust system is a concern. The conventional methods as described above, however, are insufficient to prevent the deposition of the byproducts.